Stabilization system for resonant cavity excitation

ABSTRACT

A stabilization system for resonant cavity excitation of a particle accelerator or particle storage ring includes a pair of cavity current control tubes coupled in parallel. One tube is coupled to a beam pickup in the accelerator to provide a cavity current which is a function of the beam intensity. The current through the other tube is controlled by a comparison between the voltage across the accelerating gap and a desired voltage. The currents through each of the pair of tubes are combined to provide excitation for the cavity. Each of the pair of tubes may consist of a number of tubes in parallel. The cavity is constructed so that energy at undesired frequencies is absorbed.

[451 Jan. 25, 1972 United States Patent Kerns 3,529,259 9/1970 Holmes etal..L.........................33l/l2 [54] STABILIZATION SYSTEM FORRESONANT CAVITY EXCITATION Primary Examiner-John KominskiAttorney-Roland A. Anderson [72] Inventor: Quentin A. Kerns, Glen Ellyn,Ill.

The

[73] Assignee: United States of America as represented by the UnitedStates Atomic' Energy Commission Jan. 29, 1970 cle accelerator orparticle storage ring includes a pair of cavity [22] Filed:

current control tubes coupled in parallel. One tube is coupled to a beampickup in the accelerator to provide a cavity current [2]] Appl. No.:

which is a function of the beam intensity. The current through the othertube is controlled by a comparison between the volt- [52] U.S. [51] Int.[58] Field of 5 Claims, 2 Drawing Figures UNITED STATES PATENTS 1/1961Hansen et a1.

MODULH 7 0)? P0 WER INTEGER 7'01? REFEPEA E VOL 756E MODULfl 70/?STABILIZATION SYSTEM FOR RESONANT CAVITY EXCITATION CONTRACTUAL ORIGINOR THE INVENTION The invention described herein wasmade in the courseof, or under, a contract with the UNITED STATES ATOMIC ENERGYCOMMISSION.

BACKGROUND OF THE INVENTION Advances in particle accelerator and storagering technology have led to continued increases in the magnitude of beamcurrent to be accelerated or stored. Accordingly, the disturbing effectof the charged particle beam in traversing the accelerating cavity gaphas become greater with each increase in particle beam current. Afurther problem is that the beam current often is not continuous butoccurs in interrupted pulse trains. The beam-loading effect is thus notconstant and leads to RF cavity voltage and phase fluctuations.

Changes in real loading at the gap will tend to change the amplitude ofthe voltage, while changes in reactive loading will tend to change bothamplitude and phase by introducing detuning of the resonant system. Thusthere is a requirement for rapid automatic control of both real andreactive effects. In addition, it is necessary to provide a constantradiofrequency current sufficient to balance the total resonant circuitlosses exclusive of the beam. These losses include conductor losses,dielectric losses, power tube and transmission line losses and arecharacterized by being constantly present independent of beam load.

As is well known, all resonant cavities can resonate not only at alowest mode frequency but at a large number (theoretically infinite) ofhigher frequencies. The standing wave patterns of the various high-ordermode resonances are each unique and distinguishable such that a separateindividual attack on each undesired resonance is prohibitively costly.

It is therefore an object of this invention to provide an improvedstabilization system for resonant cavity excitation.

Another object of this invention is to provide a controlled RF voltageacross a gap despite random or periodic variations in real or reactiveloading at the gap.

Another object of the invention is to provide a constant radiofrequencycurrent to balance constant resonant circuit losses of a particle beamacceleration system.

Another object of the invention is to provide -a control system whichwill cause the cavity gap voltage to followa prescribed program offrequency and voltage with time.

Another object of the invention is to limit the resonantbuildup of RFvoltages in the cavity at high-order mode frequencies.

SUMMARY OF THE INVENTION In practicing this invention, a resonant cavityis used to provide an RF voltage across an accelerating gap of aparticle accelerator or storage ring. A stabilization circuit forthecavity is provided to counteract beam loading effects and to regulatethe accelerating voltage as desired. A pair of cavity current controltubes are coupled in parallel and the current through each tube iscombined and coupled to the cavity-amplifying tube. The voltagedeveloped at the accelerating gap is a function of the combined currentsfrom each of the cavity current control tubes. One of the cavity currentcontrol tubes is coupled to a beam pickup in the accelerator to providea cavity current which is a function of the beam intensity. The currentthrough the other cavity current control tube is controlled by an errorsignal which is derived by comparing the voltage across the acceleratinggap with a desired reference voltage. Each of the cavity current controltubes can include more than one tube connected in parallel in order toprovide the desired transconductance.

The cavity is constructed as a' section of a transmission'line with oneend terminated at the accelerating gap and the other end terminated'in aresistance such as a ferrite. By correctly choosing the type of ferriteterminating the transmission line,

the energy at frequencies higher than the desired frequency is absorbed.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTIONReferring to FIG. 1, there is shown a partial schematic and partialblock diagram of the circuit of this invention. A cavity structureincludes a cavity gap 11 and a beam pickup 13.

, These structures are known and examples are illustrated in FIG. 2.Power is supplied to the cavity from power supply 14 through modulator16, choke 17 to anode 19 of tetrode 20. A plurality of control tetrodes122 and 23 are coupled in parallel between cathode 24 of tetrode 20 andground. Control grid 25 of tetrode 23 is connected to the beam pickup 13of cavity 10.

A voltage sample from the cavity gap 11 is coupled to comparator 29through capacitor 28. In comparator 29 the voltage sample is comparedwith the output of frequency synthesizer 31 in a known manner to developan error signal which is a function of the difference between thevoltage at cavity gap 11 and the desired voltage from frequencysynthesizer 31. The error signal is integrated in integrator 32 toremove short-term fluctuations and the resulting integrated error signalis applied to modulator 34.

A signal from frequency synthesizer 31 is coupled to modulator 34 andthe output of modulator 34 is coupled to linear amplifier 35. The outputof linear amplifier 35 is coupled to the control grid 26 of tetrode 22to control the current through tetrode 22. With no error signal present,the current through tetrode 22 is controlled by the output fromfrequency synthesizer 31. With an error signal present, the error signalacts to modulate the signal from modulator 34 in a manner to counteractthe error. This modulated control signal is applied to linear-amplifier25 of control grid 26 of tetrode 22. Program generator 36 connected tofrequency synthesizer 31 and modulator l6 acts to control the operationof the resonant cavity excitation.

In operation, tetrodes 22 and 23act to stabilize the resonant cavitysystem. Fluctuations in beam current are detected at the beam pickup l3and act to control the current flow through=tetrode 23. Voltagefluctuations at the cavity, gap act to control-the current flow throughtetrode 22. Since normally there are desired voltage fluctuations acrosscavity gap 11, the voltage atthis point must be compared to the desiredvoltage in order to detect errors. Any detected error is used to controlthe current flow through tetrode 22.

The currents through tetrodes 22 and 23 are combined at cathode 24 oftetrode20 and thus the flow of current through tetrode 20 is controlledby both the beam current and departures from norm of the voltage at thecavity gap 11. The beamloading effect on cavity 10 is neutralized bycontrolling the flow of current through tetrode 23 and the cavity gapvoltage is made to follow a prescribed program of frequency and voltagewith respect to time and any errors in this program areautomatically'corrected.

Referring to FIG. 2, there is shown a drawing of the mechanicalarrangement of the cavity and the portion of the particle acceleratorassociated therewith. The cavity 40 is designed as a section of auniform coaxial transmission line. A power tetrode 41 acts to supply theaccelerating voltage to gap 49 in the particle accelerator. A pluralityof tubes 43 and 44 connected in parallel are connected in series withpower tetrode 41. Thus the current flowing through tubes 43 and 44 iscombined in the power tetrode 41 to providethe necessary drive currentfor the cavity. Vacuum tube 41 corresponds to vacuum tube'20, vacuumtubes 43 correspond to vacuum tube 22, and vacuum tubes'44 correspond tovacuum tube 23 of FIG. 1. Tubes 22 and 23 can each be a single tube or anumber of tubes in parallel in order to provide the necessarytransconductance.

The particle accelerator tube 46 has an accelerating gap 49 herein. Thisgap acts as a termination for one end of the cavity and the voltagedeveloped across this gap is the accelerating voltage for the particleaccelerator. A probe 52 corresponding to capacitor 28 of FIG. 1 sensesthe voltage across the accelerating gap. A beam pickup probe 47 insertedin the particle accelerator tube 46, by developing a voltageproportional to beam current, senses the intensity of the particle beamin the accelerator tube. Beam pickup probe 47 is connected to vacuumtubes 44 and an RF drive voltage from linear amplifier 35 of FIG. 1 isconnected to tubes 43. These connections are shown in schematic form inFIG. 1. The other end of the cavity 40 is terminated with ferrite damperblocks which provide a resistive termination to absorb energy atfrequencies above 'the fundamental frequency.

The cavity is designed as a section of uniform transmission line. Forits fundamental frequency the length of the transmission line is suchthat it acts as if it were terminated in a short circuit at one end withthe accelerating gap at the other end. For higher frequencies, thecavity is no longer terminated in a short circuit but is terminated in aresistance equal to the characteristic impedance of the coaxial lineforming the cavity. This terminating resistance is provided by theferrite damper blocks. Thus higher frequency energy introduced bydisturbances at the accelerating gap flows along the transmission lineand is absorbed by the terminating resistance. If the ferrite isselected to have at the higher mode frequencies a constant surfaceimpedance in ohms/square w/ 377 ohms the complete spectrum of highermode energy will be absorbed. That is, the terminating resistor has thecharacteristics of a black body, absorbing energy at all wavelengths.

lclaim:

1. An IF stabilization circuit, including in combination, an RF resonantsystem having a cavity and a beam tube with a particle beam therein,said beam tube including a gap coupled to said cavity with said gaphaving a gap voltage thereacross for acting on said particle beam, firstcavity current control means including beam pickup means positioned insaid beam tube and responsive to the intensity of said particle beam todevelop a first control signal, second cavity current control meansincluding voltage pickup means positioned at said gap and responsive tosaid gap voltage to develop a second control signal, said first cavitycurrent control means being responsive to said first control signal todevelop a first supply current and said second cavity current controlmeans being responsive to said second control signal to develop a secondsupply current, combining means coupled to said first and second cavitycurrent control means for combining said first and second supplycurrents to develop a combined supply current, said combining meansfurther being coupled to said RF resonant system to provide saidcombined supply current to said cavity, said cavity acting to developsaid gap voltage as a function of said combined supply current.

2. The stabilization circuit of claim I further including, means forgenerating an accelerating control voltage coupled to said cavity, saidcavity being responsive to said accelerating control voltage and saidcombined supply currents to provide said gap voltage, said second cavitycurrent control means including comparator means coupled to said gap andsaid means for generating an accelerating control voltage and beingresponsive to said accelerating control voltage and said second controlsignal to develop an error signal as a function of the differencetherebetween, said second cavity current control means being responsiveto said error signal to control the magnitude of said second supplycurrent.

3. The stabilization circuit of claim 3 wherein, said second cavitycurrent control means includes integrating means for integrating saiderror signal, said second cavity current control means being responsiveto said integrated error signal to control the magnitude of said secondsupply current.

4. The stabilization circuit of claim 3 wherein, said first and secondcavity current control means include first and second vacuum tubesrespectively each having a cathode connected to a reference potential,an anode and a control electrode, said control electrode of said firsttube being coupled to said beam pickup means for receiving said firstcontrol signal, said control electrode of said second tube being coupledto said integrating means for receiving said integrated error signal,said combining means being a third vacuum tube having a cathode coupledto said anodes of said first and second vacuum tubes and an anodecoupled to said cavity.

5. The stabilization system of claim 4 wherein, said cavity is in theform of a section of a uniform coaxial transmission line terminated bysaid gap at one end and a resistance at the other end, the length ofsaid transmission line and the magnitude of said resistance being chosenso that said other end thereof acts as short circuit at the frequency ofinterest, and so that at frequencies higher than said frequency ofinterest said transmission acts as a line terminated in a resistanceequal to the characteristic impedance of said transmission line.

1. An IF stabilization circuit, including in combination, an RF resonantsystem having a cavity and a beam tube with a particle beam therein,said beam tube including a gap coupled to said cavity with said gaphaving a gap voltage thereacross for acting on said particle beam, firstcavity current control means including beam pickup means positioned insaid beam tube and responsive to the intensity of said particle beam todevelop a first control signal, second cavity current control meansincluding voltage pickup means positioned at said gap and responsive tosaid gap voltage to develop a second control signal, said first cavitycurrent control means being responsive to said first control signal todevelop a first supply current and said second cavity current controlmeans being responsive to said second control signal to develop a secondsupply curRent, combining means coupled to said first and second cavitycurrent control means for combining said first and second supplycurrents to develop a combined supply current, said combining meansfurther being coupled to said RF resonant system to provide saidcombined supply current to said cavity, said cavity acting to developsaid gap voltage as a function of said combined supply current.
 2. Thestabilization circuit of claim 1 further including, means for generatingan accelerating control voltage coupled to said cavity, said cavitybeing responsive to said accelerating control voltage and said combinedsupply currents to provide said gap voltage, said second cavity currentcontrol means including comparator means coupled to said gap and saidmeans for generating an accelerating control voltage and beingresponsive to said accelerating control voltage and said second controlsignal to develop an error signal as a function of the differencetherebetween, said second cavity current control means being responsiveto said error signal to control the magnitude of said second supplycurrent.
 3. The stabilization circuit of claim 3 wherein, said secondcavity current control means includes integrating means for integratingsaid error signal, said second cavity current control means beingresponsive to said integrated error signal to control the magnitude ofsaid second supply current.
 4. The stabilization circuit of claim 3wherein, said first and second cavity current control means includefirst and second vacuum tubes respectively each having a cathodeconnected to a reference potential, an anode and a control electrode,said control electrode of said first tube being coupled to said beampickup means for receiving said first control signal, said controlelectrode of said second tube being coupled to said integrating meansfor receiving said integrated error signal, said combining means being athird vacuum tube having a cathode coupled to said anodes of said firstand second vacuum tubes and an anode coupled to said cavity.
 5. Thestabilization system of claim 4 wherein, said cavity is in the form of asection of a uniform coaxial transmission line terminated by said gap atone end and a resistance at the other end, the length of saidtransmission line and the magnitude of said resistance being chosen sothat said other end thereof acts as short circuit at the frequency ofinterest, and so that at frequencies higher than said frequency ofinterest said transmission acts as a line terminated in a resistanceequal to the characteristic impedance of said transmission line.